1. Field of the Invention
The present invention relates to an apparatus for diagnosing a thermostat trouble in an engine cooling system.
2. Description of the Background Art
In the engine cooling system, a radiator and a thermostat (a thermostat valve) are provided. This thermostat is opened or closed in response to a cooling water temperature. It is opened when the cooling water temperature for the engine becomes more than a predetermined temperature (ex. 80.degree. C.), whereby the cooling water flows through a radiator to be cooled by the radiator. The thermostat is closed when the cooling water temperature becomes less than the predetermined temperature, whereby the cooling water bypasses the radiator so that the cooling water temperature may rise rapidly.
However, the cooling water temperature will not rise rapidly if the thermostat is caused to remain open (an opening lock) due to a problem with the thermostat. In this case the cooling water flows through the radiator and thus the engine would be actuated under a cold condition. This condition is not suitable for keeping the engine operation stable nor for improving specific fuel consumption and taking measures regarding exhaust gas.
To determine whether the thermostat trouble has occurred, Japanese Public Disclosure No.Hei 10-184433 discloses the thermostat trouble detecting device for detecting whether the actual cooling water temperature is less than the predetermined temperature a predetermined number of hours after the engine starts, namely, whether the opening trouble has occurred.
However, the above device is likely to cause the incorrect determination that a opening trouble has occurred even though actually the thermostat is not out of order, since the cooling water temperature does not rise very much even if the cooling water is not cooled by the radiator when an operating condition under which there is a small heat amount from the engine such as an idling operation continues after the engine starts.
To prevent the above incorrect determination, the present inventors developed an apparatus that determines that the opening trouble has occurred when an integrating value of deviation between the predicted cooling water temperature and the actual cooling water temperature detected by a temperature sensor is more than the predetermined temperature at the point that the cooling water temperature predicted based on the engine operating condition reaches the predetermined temperature. According to the device for determining, the incorrect determination that the thermostat opening trouble has occurred can be prevented since it takes a longer time until the predicted temperature reaches the predetermined temperature even if an operating condition under which there is a small heat amount from the engine such as an idling operation continues after the engine starts.
However, in the device for determining using the predicted temperature as described above, the predicted temperature reaches the predetermined temperature for a short period if an operating condition under which the cooling water temperature rises rapidly continues, for example, if the acceleration of a vehicle up a steep ascent continues directly after the engine starts, and thus the determining device is likely to determine incorrectly that the thermostat is normal since the integrating value becomes small even if the opening trouble occurs.